A separator comprises a porous film having micropores, and it is used for a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery and a lithium polymer secondary battery. In the non-aqueous electrolyte secondary battery, it is important to interrupt an electric current and prevent excessive flow of the electric current (shutdown), when an abnormal current flows in the battery due to an electrical short circuit between a cathode and an anode. Therefore, the separator is required to shut-down the current (to plug the micropores of the porous film) at a temperature as low as possible, when the temperature rises exceeding a normal operating temperature, and even if the temperature inside the battery rises to a certain high temperature after the current is shut-down, not to break the film due to the high temperature and to maintain the shut-down state as it is, in other words, to have high heat-resistance.
Examples of conventional separators include a separator made of a laminated porous film in which a polyolefin layer and a heat-resistant layer are laminated. As a specific example of such a separator, JP-A-2005-285385 and JP-A-2006-032246 describe a separator produced by coating one side of a polyethylene film with a solution prepared by dissolving a polyamide as a heat-resistant material in N-methyl-2-pyrrolidone as a water-soluble solvent, then immersing the coated polyethylene film in water to remove N-methyl-2-pyrrolidone and to deposit and coagulate the polyamide, and drying the film.